While both sperm motility and morphology are usually evaluated as indications of fertility, their relative importance in fertility is unknown. In many cases of infertility abnormal sperm morphology is correlated with poor motility. Sperm morphology has been shown to be genetically controlled and different strains of mice show characteristic differences in the size and shape of the sperm head, as well as different levels of morphologically abnormal sperm. These characteristics of sperm morphology are usually established between 6 and 10 wks. of age and are maintained until old age. No systematic studies on the influence of genotype on the pattern of sperm motility have been reported and the timing of the acquisition of normal patterns of sperm motility is also unknown. Many mutant genes lead to infertility in male but not female mice. These mutant mice provide the opportunity to study the characteristics of fertile and infertile sperm. This study has three aims. The first is to investigate differences in the motility patterns of normal sperm of different sizes and shapes from different strains of mice. These data should enable us to determine the correlation between morphology and motility. They will also provide the starting point for the analysis of the genetic influence on sperm motility. The second goal is to compare the swimming characteristics of morphologically abnormal sperm to normal sperm from the same animal. These studies will demonstrate whether certain types of morphological alterations are associated with motility impairment. The third goal is to determine the morphology and motility of sperm from mutant males that have unexplained infertility. The comparison of morphology and motility characteristics of infertile and fertile males may suggest which characteristics of sperm are most essential for fertility.